This type seems to cover the intent more clearly than `std::pair<bool,
...>`.

This should only end up on the nightly wheel upload for now.

Also, re-enable testing in places where Pillow wheels were previously
missing, but are now available.

Closes #29688

Objects without a parent are not necessarily cleaned up in `PyQt5/6`
when their reference count reaches zero, and must be explicitly cleaned
up with `deleteLater()`

This prevents the notifier firing after the signal handling was supposed
to have been reset to its previous state.

Rather than have both `bakend_bases._allow_interrupt()` and
`backend_qt._allow_interrupt_qt()` hold a reference to the notifier, we
pass it to  the backend-specific `handle_signint()` function for
cleanup.

Note the approach to cleaning up the notifier with `.deleteLater()`
followed by `sendPostedEvents()` is the documented workaround for when
immediate deletion is desired:

https://doc.qt.io/qt-6/qobject.html#deleteLater

This ensures the object is still deleted up even if the same event loop
does not run again.

ci: Enable wheel builds on Python 3.14

Clean up Qt socket notifier to avoid spurious interrupt handler calls

It can like be made even clearer; here I tried to just repeat what
was written but with a clearer style.

Some minor additional edits are included as well.

agg: Replace facepair_t with std::optional

Copy-edit the docstring of AuxTransformBox.